Optical communication systems transmit data using electromagnetic light signals in optical fiber and/or free space (for example, building to building, ground to satellite, satellite to satellite, etc.). The electromagnetic carrier wave is modulated to carry the data. Optical communication in optical fiber typically involves: generating the optical signal, relaying the signal on an optical fiber (including measures to reduce/mitigate attenuation of, interference with and/or distortion of the light signal), processing a received optical signal, and converting the signal into a useful electrical signal. Transmitters can be semiconductor devices such as laser diodes, producing coherent light for transmission. A number of receivers have been developed for processing a transmitted lightwave optical signal to provide processed optical signal input(s) to one or more photodetectors, which convert light into electricity.
A coherent receiver, such as an Integrated Coherent Optical Receiver (ICR), converts a modulated optical signal into four electrical signals corresponding to an “in-phase” (I) and “quadrature” (Q) optical signal components of the two optical polarization states, vertical and horizontal. These components can be processed to recover the optically transmitted data regardless of modulation type. Thus, the four output electrical signals from the ICR carry all or nearly all of the information conveyed by the optical signal.
Testing an ICR presents a special challenge in that the output stage is a balanced detector pair often followed by a differential amplifier with differential outputs. The fact that there are four differential outputs (I and Q each for X and Y polarizations), compounds the difficulty. A simple coherent receiver is composed of a local-oscillator laser, an optical coupler, and one or more photodetectors that can be in a “balanced” configuration that cancels photocurrents and eliminates DC terms and the related excess intensity noise.
The balanced detection and differential amplification of the ICR ensure that any signal put into only the signal port or only the Local Oscillator (LO) port of the ICR will be rejected unless it is possible to block one of the photodiodes to break the balanced detection. Although early versions of ICRs allowed physical access to interrupt a light signal and thereby break the balanced detection, this is not possible on modern integrated components, which are instead typically intrinsically sealed. Getting any meaningful signal out of the ICR therefore requires both a signal and a LO input. This requirement can complicate some desired measurements to be performed on a Device Under Test (DUT), where the optical LO input must be phase coherent with the test signal input.
Because the ICR requires both a signal and a local oscillator input to provide meaningful output, the frequency and phase relationship between the two input signals are important. While it is simplest to split the LO and Signal lasers and then connect them to a reference coherent receiver front end and a Device Under Test (DUT), the separate fiber paths required by this configuration can introduce an unknown phase difference between the input signals.
Embodiments of the present invention determine, correct for, and/or control a phase difference between the local oscillator signal and the test signal input to a DUT. This ability can be used to restore the phase coherence needed for certain desired performance tests of the DUT.